Optical diodes which change the quantity of transmitted light depending on the direction of light incident thereon have conventionally been known (see, for example, the following Patent Document 1). This document proposes an optical diode consisting of a laminate in which low-refractive-index layers and high-refractive-index layers are alternately laminated, the refractive indexes of the low-refractive-index and high-refractive-index layers gradually increase along the thickness direction of the laminate, and the low-refractive-index or high-refractive-index layers have a nonlinear optical characteristic.
It is supposed that, when laser light having such a high intensity as to cause a nonlinear optical effect in a low-refractive-index or high-refractive-index layer is incident thereon, this optical diode partly transmits the light incident on one side of the laminate but substantially reflects the light incident on the other side.
Patent Document 1: U.S. Pat. No. 5,559,825